// Receiving a GOAWAY frame will cause the Http2Session to first emit a 'goaway'
// event notifying the user that a shutdown is in progress. If the goaway
// error code equals 0 (NGHTTP2_NO_ERROR), session.close() will be called,
// causing the Http2Session to send its own GOAWAY frame and switch itself
// into a graceful closing state. In this state, new inbound or outbound
// Http2Streams will be rejected. Existing *pending* streams (those created
// but without an assigned stream ID or handle) will be destroyed with a
// cancel error. Existing open streams will be permitted to complete on their
// own. Once all existing streams close, session.destroy() will be called
// automatically.